1. Field of the Invention
The present invention relates to a confocal optical device, a spherical-aberration correction method, a film-thickness measurement apparatus, a film-thickness measurement method, a shape measurement apparatus and a shape measurement method.
2. Description of the Related Art
A confocal optical device has a precise resolving power in the directions of an optical axis. Hence, it is used for three-dimensional measurement of transparent subjects or laser microscopy. For example, it is used to measure the thickness of a cover-glass layer in an optical disk, or measure the thickness of a cover-glass layer and an intermediate layer in a double-layer optical disk.
A conventional confocal optical device will be described using FIG. 9. FIG. 9 shows the configuration of a conventional confocal optical device which is described in Japanese Patent Laid-Open No. 5-134186 specification. As shown in FIG. 9, a beam of light which is emitted from a laser light source 41 passes through a light-branching element 42. Then, it concentrates through a first lens group 43 and passes through a pinhole 44. This pinhole 44 is provided at the focal point of the first lens group 43. Then, the beam of light which has passed through this pinhole 44 is incident upon a beam expander 45.
The beam expander 45 includes a second lens group 45a, drive means 45d, 45e, 45f, and a motor 45g. The second lens group 45a includes a concave lens 45b and a convex lens 45c. Then, in the beam expander 45, the position of each lens which makes up the second lens group 45a is shifted by the drive means 45d, 45e, 45f, and the motor 45g. Thereby, a beam of light which is emitted from the beam expander 45 is designed to diverge or converge. The beam of light which has passed through the beam expander 45 is reflected by a mirror 46. Then, it passes through a scanning optical system 47 and is incident upon an objective lens 48. The beam of light which has come into this objective lens 48 concentrates upon the inside of a sample 49 which is fixed on a sample board 50. Herein, the scanning optical system 47 is used to scan a beam on the perpendicular plane to the optical axis.
The beam of light which is reflected from the inside of the sample 49 propagates in the direction opposite to what is described above in the optical system. Then, it passes through the pinhole 44. At this time, only the reflected beam of light from the focal point of the objective lens 48 and its vicinity passes through the pinhole 44. This is because a confocal optical system has such a function, which is generally known. The beam of light which has passed through the pinhole 44 branches off at the light-branching element 42. Then, it is detected by a photoelectric detection element 51. The photoelectric detection element 51 outputs an electric signal according to the quantity of a beam of light which it receives.
The position of each lens of the above described beam expander 45 is changed, so that a beam of light incident upon the objective lens 48 can be turned into a divergent beam of light or a convergent beam of light. Thereby, the point of a beam of light which is concentrated by the objective lens 48 can be scanned in the optical-axis directions inside of the sample 49. Specifically, if the second lens group 45a is moved to the side of the pinhole 44, an incident beam of light upon the objective lens 48 becomes a divergent beam of light. Thus, the concentrated-light point moves to the side away from the objective lens 48. On the other hand, if the second lens group 45a is moved to the side of the objective lens 48, an incident beam of light upon the objective lens 48 becomes a convergent beam of light. Thus, the concentrated-light point moves to the side close to the objective lens 48. This makes it possible to observe the inside of the sample 49 from its deep part to the shallow part. In this case, if a beam of light incident upon the objective lens 48 is transformed into a divergent beam of light or a convergent beam of light, a spherical aberration is produced. But this spherical aberration is designed to be offset by changing the distance between the concave lens 45b and the convex lens 45c which make up the second lens group 45a. 
However, in the confocal optical device according to the above described Japanese Patent Laid-Open No. 5-134186 specification, there is still room to correct a spherical aberration more precisely. Specifically, in this confocal optical device, a beam of light incident upon the objective lens 48 is turned into a divergent beam of light or a convergent beam of light. Thereby, the position of a concentrated-light point of the incident beam of light varies according to the depth directions inside of the sample 49. Therefore, the spherical aberration which is generated when a beam of light is transmitted into the sample 49 changes, depending upon the depth at which the concentrated-light point is located in the sample 49. In such a confocal optical device, the fact that a spherical aberration is affected by the depth inside of the sample 49 is left out of account. Hence, when the focal point is brought to a desired depth, this spherical aberration which is affected by the depth inside of the sample 49 cannot be reduced.